Method of forming composite graphite coated article



March 9, 1965 R. J. DIEFENDORF 3,172,774

METHOD OF FORMING COMPOSITE GRAPHITE COATED ARTICLE Filed Feb. 28, 1961Inventor: Russell J. D/efendorf b, m WWI His A from e y United StatesPatent 3,172,774 METHOD OF F0 l G COMPOSITE GRAP COATED ARTICLE RussellJ. Diefendorf, Schenectady, N.Y., assignor to General Electric Company,a corporation of New York Filed Feb. 28, 1961, Ser. No. 92,417 7 Claims.(Cl. 11746) This invention relates to methods of formin compositearticles and more particularly to methods of forming composite anticleshaving a member and a pyrolytic graphite coating.

This application is a continuation-in-part of my copending applicationfiled January 3, 1961, as Serial No. 80,080, now abandoned.

Pyrolytic graphite is defined as a polycrystalline material made fromcarbonaceous gases by thermal decomposition or from a carbonaceousmaterial by evaporation and deposition on a surface in which the planargraphite crystallites are aligned into a layer structure. It is usefulas a high temperature material for lamp filaments, furnace linings andneutron reactor moderators. Development of missile and space propulsionsystems has created an additional requirement for composite pyrolyticgraphite components in these systems.

The thermal expansion coefiicients of a piece of pyrolytic graphite aremeasured along the C direction and the A direction, a directiongenerally perpendicular to the C direction. The C direction is definedas the component of the maximum measured number of crystallites in the Cdirection or orientation. The A direction is defined as the component ofthe maximum measured number of crystallites in directions perpendicularto the C direction. The expansion coeflicient of a pyro lytic graphitecoating measured along its C direction is in the range of about 10.0 10-to 28.5 10 per centigrade degree. The average expansion coeflicientalong its A direction, an axis generally perpendicular to its Cdirection, is in the range of about 0.8 10 to 3.0x l0 per centigradedegree from room temperature to 1000 C.

Carbonaceous gases have been thermally decomposed and deposited onsurfaces to produce pyrolytic graphite. As a result of thedecomposition, carbon is removed from the gas and deposits on thesurface so that planar graphite crystallites are aligned into a layerstructure. It is desirable to provide composite pyrolytic graphitearticles at high deposition rates in which each article includes apyrolytic graphite coating bonded tightly to a member. Furthermore, itis advantageous to have a coating which has a similar coefficient ofexpansion to its member at room temperature. It would appear that onlythe temperature need be increased in the deposition chamber to produce.a corresponding increase in deposition rate. However, in a depositionat a temperature above 1000 C., the pyroly-tic graphite coating isapplied rapidly but the expansion coefiicient of the coating in theplane of deposition or A direction is substantially less than theexpansion coefficient of the member in the same direction with resultingpeeling or popping off of the coating upon cooling the article to roomtemperature. Thus, a temperature increase does not solve the depositionproblem but provides a non-adhering layer which does not produce acomposite article. Therefore, it would be desirable to provide methodsof forming composite pyrolytic graphite articles at high depositionrates.

3,172,774 Patented Mar. 9, 1965 It is an object of my invention toprovide a deposition method of forming composite pyrolytic graphitearticles.

It is another object of my invention to provide a deposition method offorming composite pyrolytic graphite articles in which the coating hassoot particles deposited uniformly therein.

It is a further object of my invention to provide a deposition method offorming pyrolytic graphite articles of uniform thickness at high ratesof deposition.

In carrying out my invent-ion in one form, a member is positioned withina chamber, a carbon vapor is flowed at a temperature in the range of1450 C. to 2000 C. through the chamber, and the pressure is maintainedwithin the chamber at a pressure in the range of .2 centimeter ofmercury to centimeters of mercury to form a composite article.

These and various other objects, features, and advantages of theinvention will be better understood from the following description takenin connection with the accompanying drawing in which:

FIGURE 1 is a sectional view of a deposition apparatus for formingcomposite articles in accordance with my invention; and

FIGURE 2 is a sectional view of a modified deposition apparatus.

In FIGURE 1, a deposition apparatus is shown generally at 10 whichcomprises a chamber 11 having a lower body portion 12 and a cover 13which is hinged to the lower body portion by means of bolts 14 andemploys an O-ring 15 therebetween. Viewing window 16 is provided incover 13 to View the operation and to read an optical pyrometer (notshown). A feed line 17 extends through the side wall of chamber 11 andis connected to a carbonaceous material source (not shown). For example,a carbonaceous gas is fed from the source through a meter 18 showing thetotal consumption of gas, a gas rate meter 19, an acetone and Dry Icetrap indicated at 20, a preheater 21, and line 17 to chamber 11. While apure carbonaceous gas, such as methane, ethane, propane, acetylene,benzene, carbon tetrachloride, or cyanogen, is employed, thecarbonaceous material can also be in liquid or solid form which is fedfrom the source to preheater 21. It is desirable to heat thecarbonaceous material in preheater 21 to a temperature in the range ofroom temperature to 2000 C. to convert the material to its gaseous stateor to decompose the material to a carbon vapor.

An insulated cylinder 22 of quartz or alumina is surrounded byconventional heating coils 23. Suitable insulation in the form of carbonblack 24 surrounds a member 25 of graphite or other high temperaturematerial with an exposed surface 26. Coils 23 provide heat for member 25and chamber 11 during the deposition process. Chamber 11 is alsoprovided with an outlet 27 to which is connected a line 28 associatedwith a vacuum pump 29 to maintain a low pressure in the chamber.

In FIGURE 2 of the drawing, a chamber 11 is shown which has a lower bodyportion 12 and a cover 13 hinged to portion 12 by means of bolts 14. AnO-ring 15 is employed between portion 12 and cover 13. Viewing window 16is provided in cover 13 to View the operation and to record thetemperature by means of an optical pyrometer (not shown). A feed line 17extends through the bottom wall of chamber 11 and is connected to acarbonaceous material source (not shown). For example, a carbonaceousgas is fed from the source through total consumption meter 13, gas ratemeter 1%, acetone and Dry Ice trap 20, preheater 21, and. line 17 tochamber 12. An insulated cylinder 22 of quartz or alumina is surroundedby heating coils 23. For example, a member 30 in the form of a sheet ofgraphite is suspended by any suitable means, such as a hook 31 and rod32 within cylinder 22 of chamber 11. If it is desired, cylinder 22 andheating coils 23 can be removed and heat provided only in preheater 21to produce the carbon vapor for deposition.

I discovered unexpectedly that composite pyrolytic graphite articleswere formed with uniform soot deposits in the coatings providingadherence to the member by positioning a member within a chamber,flowing a carbon vapor at a temperature in the range of 1450 C. to 2000C. through the chamber, and maintaining within the chamber a pressure inthe range of .2 centimeter of mercury to 70 centimeters of mercury. Ifound further that the coatings of these composite articles were hardand more isotropic whereby the coating adhered tightly to the member.Additionally, the coefiicients of expansion of both the coating andmember were more similar at room temperature.

It appears that the soot particle is probably formed by the growth of alarge carbon molecule which upon reaching a critical size is coatedaround its periphery with smaller aromatic molecules. These smallermolecules are oriented with their basal planes parallel to the surfaceof the larger molecule. If a large diameter chamber is employed at lowpressure, the large carbon molecule does not have time to difiiuse tothe chamber wall before it forms soot. An increase in the pressure ortemperature increases the soot formation. If a small diameter chamber isused at low pressure, the large carbon molecule does have time todiifuse to the chamber wall without soot formation. However, an increasein pressure or temperature in the small diameter chamber to increasedeposition produces alternate layers of soot and pyrolytic graphiterather than a coating with uniform soot deposits.

In the operation of deposition apparatus shown in FIGURE 1, cylinder 22surrounded by coils 23 is positioned within chamber 11. Carbon black 24is placed in the bottom of cylinder 22 after which member 25 ispositioned thereon. Additional carbon black 24 is placed in the voidbetween the member and cylinder leaving surface 26 of member 25 exposedto the chamber atmosphere. Cover 13 is bolted to lower body portion 12of chamber 11 and the chamber atmosphere is reduced to a pressure in therange of .2 centimeter of mercury to 70 centimeters of mercury. I preferto operate between 30 and 40 centimeters of mercury pressure. Power issupplied to induction coils 23 which heats the atmosphere of chamber 11and member 25 to a temperature in the range of 1450" C. to 2000 C. todecompose the carbonaceous material to a carbon vapor or to maintain thetemperature of the carbon vapor during deposition in the chamber.

A carbonaceous gas, such as methane, is fed through a total consumptionmeter 18, a gas rate meter 19, and an acetone and Dry Ice trap 20 priorto entering preheater 21 through gas line 17. The carbonaceous gas ispreheated in preheater 21 to a temperature in the range of roomtemperature to 2000 C. If a liquid or solid carbonaceous material isused, the material is fed to preheater 21 in which it is converted toits gaseous form. Some hydrocarbon gases will liquify in trap 20 whichliquids are then converted to gases in preheater 21. The carbonaceousmaterial can also be fed to chamber 11 where heat from coils 23decomposes the material to a carbon vapor. I have also found thatcarbonaceous material can be preheated in preheater 21- to a temperaturein the range of 1450 C. to 2000 C., to convert the material to itsgaseous state and then to decompose the gas to carbon vapor which isflowed into chamber 11. It is also desirable to heat the chamber in a.temperature range of 1450 C. to 2000 to maintain the temperature of thevapor during deposition in the chamber.

Although some of the carbon vapor will deposit on the walls of thepreheater and the chamber, most of the vapor will be deposited onexposed surface 26 of member 25. Line 28 and pump 21 remove gases andmaintain the desired pressure within chamber 11. While the above processwith a carbonaceous gas can be carried out over a range of .2 centimeterof mercury to 70 centimeters of mercury, at various gas flow rates, Iprefer to reduce the atmosphere to a pressure of 30 to 40 centimeters ofmercury and operate generally at 35 centimeters of mercury for bestresults.

After the desired thickness of pyrolytic graphite coating on thesubstrate is attained, the gas flow is stopped, the pressure isdecreased further, and the composite article within chamber 11 isallowed to cool to room temperature. The pressure is increasedsubsequently to atmospheric pressure, and cover 13 is removed to provideaccess to coated member 25 which is removed from chamber 11. During theoperation of forming such composite articles, the temperature isrecorded by an optical pyrometer (not shown) which is viewed throughwindow 16 in cover 13.

In FIGURE 2 of the drawing, there is shown a modified depositionapparatus wherein cylinder 22 surrounded by coils 23 is positionedwithin chamber 11. For example, a member 30 in the form of a sheet ofgraphite is suspended by a hook and rod 32 within cylinder 22. Cover 13is bolted to lower body portion 12 of chamber 11 and the chamberatmosphere is reduced to a pressure in the range of .2 centimeter ofmercury to 70 centimeters of mercury. Power is supplied to inductioncoils 23 which heats the atmosphere of chamber 11 and member 25 to atemperature in the range of 1450 C. to 2000 C. A carbonaceous gas, suchas methane, is fed through a total consumption meter 18, a gas ratemeter 19, and an acetone and Dry Ice trap 20 prior to entering preheater21 through gas line 17. The carbonaceous gas is preheated in preheater21 to a temperature in the range of room temperature to 2000 C. If aliquid or solid carbonaceous material is used, the material is fed topre heater 21 in which it is converted to its gaseous form.

The carbonaceous material can also be fed to chamber 11 Where heat fromcoils 23 decomposes the material to a carbon vapor. I have also foundthat carbonaceous material can be preheated in preheater 21 to atemperature in the range of 1450 C. to 2000 C., to convert the materialto its gaseous state and then to decompose the gas to carbon vapor whichis flowed into chamber 11. It is also desirable to heat the chamber in atemperature range of 1450 C. to 2000 C. to maintain the term perature ofthe vapor during deposition in the chamber. Although some of the carbonvapor will deposit on the walls of the preheater and the chamber, mostof the vapor will be deposited on member 30. Line 28 and pump 29 removegases and maintain the desired pressure within chamber 11. The aboveprocess can be carried out over a range of .2 centimeter of mercury to70 centimeters of mercury, at various gas flow rates.

After the desiredthickness of pyrolytic graphite coating on the memberis attained, the gas flow is stopped, the pressure is decreased further,and the composite article within chamber 11 is allowed to cool to roomtemperature. The pressure is increased subsequently to atmosphericpressure, and cover 13 is removed to provide access to coated member 30which is removed from chamber 11. During the operation of forming suchcomposite articles, the temperature is recorded by an optical pyrometer(not shown) which is viewed through window 16 in cover 13. If desired, aplurality of substrates can be coated simultaneously within chamber 11.

Several examples of methods of forming composite pyrolytic graphitearticles in accordance with the pres ent invention are as follows:

Example I A deposition apparatus was set up generally in accordance withFIGURE 1 of the drawing wherein the member was composed of commercialgraphite. After the cover was bolted to the lower body portion, thechamber atmosphere was reduced to a pressure of .0001 centimeter ofmercury by the pump. Power was supplied to the induction coil to heatthe chamber, and member to an uncorrected optical pyrometer temperaturereading of about 1650 C. A carbonaceous gas in the form of methane wassupplied to the chamber at an ultimate rate of 20 cubic feet per hour atan ultimate pressure of 350 mm. of mercury subsequent to flowing throughmetering devices, and an acetone and Dry Ice bath. The pressure of thegas was increased slowly so that large quantities of soot were notvisible during the pressure buildup. The carbon vapor which was formedwas deposited on the member as it flowed through the chamber. After sixhours, the power and gat flow were discontinued and the chamber wasrestored to atmospheric pressure. After cooling to room temperature, thecomposite pyrolytic graphite article was removed from the chamber. Thecoating on the article had a thickness of 30 mils.

Example II A deposition apparatus was set up generally in accordancewith FIGURE 2 of the drawing wherein the member was composed ofcommercial graphite. After the cover was bolted to the lower bodyportion the chamer atmosphere was reduced to a pressure of .0001centimeter of mercury. Power was supplied to the induction coil to heatthe chamber, and member to an uncorrected optical pyrometer temperaturereading of about 1350 C. A carbonaceous gas in the form of methane wassupplied to the chamber at an ultimate rate of eight cubic feet per hourand was bubbled through a container filled wih dicyclopentadiene at anultimate pressure of 20 mm. of mercury subsequent to flowing hroughmetering devices, and an acetone and Dry Ice bath. The pressure of thegas was increased slowly so that large quantities of soot were notvisible during the pressure buildup. The carbon vapor which was formedwas deposited on the member as it flowed through the chamber. After twohours, the power and gas flow were discontinued and the chamber wasrestored to atmospheric pressure. After cooling to room temperature, thecomposite pyrolytic graphite article was removed from the chamber. The

coating on the article had a thickness of approximately mils.

Example 111 A deposition apparatus was set up generally in accordancewith FIGURE 2 of the drawing wherein the member Was composed ofcommercial graphite. After the cover was bolted to the lower bodyportion, the chamber atmosphere was reduced to a pressure of .0001 mm.of mercury by the pump. Power was supplied to the induction coil to heatthe chamber,- and chamber to an uncorrected optical pyrometertemperature reading of about 1500 C. A carbonaceous gas in the form ofmethane was supplied at an ultimate rate of 12 cubic feet per hour at anultimate pressure of 2.3 mm. of mercury to the preheater subsequent toflowing through metering devices, and an acetone and Dry Ice bath. Thepressure of the gas was increased slowly so that large quantities ofsoot were not visible during the pressure buildup. The gas was heated ata temperature of 1500 C. and formed into a carbon vapor in the preheaterwhich vapor was deposited on the member as it flowed through thechamber. After four hours, the power and gas flow were discontinued andthe chamber was re stored to atmospheric pressure. After cooling to roomtemperature, the composite pyrolytic graphite article was 6 removed fromthe chamber. The coating of the article has a thickness of 10 mils.

While other modifications of this invention and variations of methodwhich may be employed within the scope of the invention have not beendescribed, the invention is intended to include such that may beembraced within the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A deposition method which comprises providing a chamber, positioningat least one member within said chamber, flowing a carbon vapor at atemperature in the range of l450 C. to 2000 C. through said chamber,maintaining the pressure within said chamber at a pressure in the rangeof .2 centimeter of mercury to 70 centimeters of mercury and depositinga pyrolytic graphite coating with uniform soot deposits on said memberto form a composite article.

2. A deposition method which comprises providing a chamber, positioningat least one member within said chamber, flowing a carbon vapor intosaid chamber, heating said chamber, said member, and said vapor to atemperature in the range of 1450 C. to 2000 C., flowing said carbonvapor through said chamber, maintaining the pressure within said chamberat a pressure in the range of .2 centimeter of mercury to 70 centimetersof mercury, and depositing a pyrolytic graphite coating with uniformsoot deposits on said member to form a composite article.

3. A deposition method which comprises providing a chamber, positioningat least one member within said chamber, heating said chamber and saidmember to a temperature in the range of 1450 C. to 2000 C., flowing acarbon vapor at a temperature in the range of 1450 C. to 2000 C. throughsaid chamber, maintaining the pressure within said chamber at a pressurein the range of .2 centimeter of mercury to 70 centimeters of mercuryand depositing a pyrolytic graphite coating with uniform soot depositson said member to form a composite article.

4. A deposition method which comprises providing a chamber, positioningat least one member within said chamber, feeding a carbonaceous materialto said chamber, heating said chamber, said member, and said material toa temperature in the range of 14-50" C. to 2000 C. whereby said materialdecomposes toa carbon vapor, flowing said carbon vapor through saidchamber, maintaining the pressure within said chamber at a pressure inthe range of .2 centimeter of mercury to 70 centimeters of mercury anddepositing a pyrolytic graphite coating with uniform soot deposits onsaid member to form a composite article.

5. A deposition method which comprises providing a chamber, positioningat least one member within said chamber, feeding a carbonaceous materialto said chamber, preheating said material at a temperature in the rangeof 1450 C. to 2000 C. to decompose said material to a carbon vapor,flowing said carbon vapor at a temperature in the range of 1450 C. to2000 C. through said cham ber, maintaining the pressure within saidchamber at a pressure in the range of .2 centimeter of mercury to 70centimeters of mercury, and depositing a pyrolytic graphite coating withuniform soot deposits on said member to form a composite article.

6. A deposition method which comprises providing a chamber, positioningat least one member within said chamber, heating said chamber and saidmember to a temperature in the range of 1450 C. to 2000 C., feeding acarbonaceous material to said chamber, preheating said material at atemperature in the range of l450- C. to 2000 C. to decompose saidmaterial to a carbon vapor, flowing said carbon vapor at a temperaturein the range of 1450 C. to 2000 C. through said chamber, maintaining thepressure Within said chamber at a pressure in the range of .2 centimeterof mercury to 70 centimeters of mercury and depositing a pyrolyticgraphite coating with uniform soot deposits on said member to form acomposite article.

7. A deposition method which comprises providing a chamber, positioningat least one member within said chamber, flowing a carbon vapor at atemperature in the range of 1650" C. to 1850" C. through said chamber,maintaining the pressure within said chamber at a pressure in the rangeof 30 centimeters of mercury to 40 centimeters of mercury and depositinga pyrolytic graphite coating with uniform soot deposits on said memberto form a composite article.

References Cited by the Examiner 5 UNITED STATES PATENTS 2,405,449 8/46Robinson et a1. 117-226 X 2,487,581 11/49 Palurnbo 117226 X 2,789,0384/57 Bennett et a1. 23209.4 X

RICHARD D. NEVIUS, Primary Examiner.

1. A DEPOSITION METHOD WHICH COMPRISES PROVIDING A CHAMBER, POSITIONINGAT LEAST ONE MEMBER WITHIN SAID CHAMBER, FLOWING A CARBON VAPOR AT ATEMPERATURE IN THE RANGE OF 1450*C. TO 2000*C. THROUGH SAID CHAMBER,MAINTAINING THE PRESSURE WITHIN SAID CHAMBER AT A PRESSURE IN THE RANGEOF .2 CENTIMETER OF MERCURY TO 70 CENTIMETERS, OF MERCURY AND DEPOSITINGA PYROLYTIC GRAPHITE COATING WITH UNIFORM SOOT DEPOSITS ON SAID MEMBERTO FORM A COMPOSITE ARTICLE.